Field of the Invention
The present invention relates to a wavelength plate and an optical device.
Description of the Related Art
Wavelength plates are elements configured to impart a specific retardation to light, and those that are the most commonly used are half wavelength plates and quarter wavelength plates. These wavelength plates are mounted on many optical devices.
Various studies have been conducted in order to obtain wide-band wavelength plates, and various multilayer wavelength plates obtained by laminating wavelength plates one over another have been proposed in order to obtain, for example, wide-band quarter wavelength plates. For example, there has been proposed a wavelength plate obtained by bonding three wavelength plates made of inorganic optical single crystals such as quartz crystals (see, e.g., Japanese Patent (JP-B) No. 4534706). There has also been proposed a wavelength plate obtained by bonding a quarter wavelength plate and a half wavelength plate that are made of organic materials such as stretched polymer films (see, e.g., Japanese Patent Application Laid-Open (JP-A) No. 10-68816 and JP-B Nos. 5191447 and 4708787).
However, JP-B No. 4534706 uses an adhesive for bonding the crystal plates, which raises a risk that the adhesive may deteriorate from a long time of use, which is disadvantageous in terms of heat resistance and durability. Further, the degree of retardation by the quartz-crystal wavelength plate is greatly varied depending on the angle of incident light, which raises problems that so-called angle-dependency of the wavelength plate is poor, and that the wavelength plate may not be able to adapt to expansion of the angular range of light used in recent optical devices.
Further, the stretched polymer films of JP-A No. 10-68816, JP-B No. 5191447, or JP-B No. 4708787 have a problem in durability because of a weak resistance to heat and UV light rays. This is disadvantageous for apparatuses and projectors that use a light source having a high light intensity such as a laser light source and are hence required to have resistance to heat and light.
Meanwhile, there has also been proposed a wavelength plate obtained by laminating birefringent layers formed by oblique deposition (see, e.g., JP-A No. 11-23840).
Incidentally, quarter wavelength plates are used in optical isolator optical systems, Serunamon optical systems, optical pickups, polarimetric interferometers, reflective liquid crystal projectors, etc. Among these, optical isolator optical systems, polarimetric interferometers, and reflective liquid crystal projectors make linearly polarized light that has once transmitted through the quarter wavelength plate be reflected on a mirror, a liquid crystal panel, or the like and make the reflected light transmit through the same quarter wavelength plate again. That is, these devices make the quarter wavelength plate function as a half wavelength plate by making light pass through the quarter wavelength plate twice. By this, when linearly polarized light becomes incident, these devices aim to obtain the linearly polarized light that has been rotated by 90° through two passages before and after the reflection. However, a wavelength plate that functions as, for example, a wide-band quarter wavelength plate may not necessarily be able to efficiently obtain linearly polarized light that has been rotated by 90° by two passages through the wavelength plate. Hence, in consideration of application to optical isolator optical systems and the like, there is a need for a wavelength plate having a high emission rate of linearly polarized light that has been rotated by 90° when linearly polarized light has passed through the wavelength plate twice in a reciprocating manner, i.e., a wavelength plate having a good conversion efficiency.
However, there has not been provided a wavelength plate that overcomes all of the problems described above, and even the wavelength plate of JP-A No. 11-23840 mentioned above cannot be said to be satisfactory in capability of exhibiting a high conversion efficiency (see the result of Comparative Example 2 in which an experiment was conducted based on the embodiment of JP-A No. 11-23840 in the EXAMPLES section below). Moreover, JP-A No. 11-23840 has a risk of cost increase because two or more kinds of birefringent materials are required, and selectivity of the materials is limited because there is a regulation for the relationship between wavelength dispersion and retardation of each birefringent material.
Hence, there is a need for an inorganic wide-band quarter wavelength plate having excellent heat resistance, a high conversion efficiency when light has passed through the wavelength plate twice in a reciprocating manner, and a small variation in the degree of retardation depending on the angle of incident light.